[Proj] Using cs2cs.

[Joshua Brown]

On 30/04/12 13:03, Hermann Peifer wrote:
> On 30/04/2012 11:27, Joshua Brown wrote:
>> On 30/04/12 07:57, Hermann Peifer wrote:
>>> On 30/04/2012 05:54, Joshua Brown wrote:
>>>> Hello,
>>>>
>>>> I have a text file, with each OSGB32 value on a new line.
>>>>
>>>> I want to convert from OSGB32 to WGS84 so I can map values to google
>>>> earth.
>>>>
>>>> How can I use cs2cs to convert the text file into WGS84?
>>>>
>>>
>>> I assume that with OSGB32 you actually mean: OSGB36. I have recently
>>> been using [1] and [2] for getting from OSGB36 to WGS84. [2] is the more
>>> accurate alternative. You can find a link to OSTN02_NTv2.gsb at the PROJ
>>> wiki page, see [3].
>>>
>>> Hermann
>>>
>>> [1]
>>> cs2cs -v -f "%.6f" +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
>>> +x_0=400000 +y_0=-100000 +ellps=airy
>>> +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +to
>>> +proj=longlat +datum=WGS84
>>>
>>> [2]
>>> cs2cs -v -f "%.6f" +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
>>> +x_0=400000 +y_0=-100000 +ellps=airy +nadgrids=./OSTN02_NTv2.gsb  +to
>>> +proj=longlat +datum=WGS84
>>>
>>> [3] http://trac.osgeo.org/proj/wiki
>>
>> Thanks for the response,
>>
>> Yes I do mean OSGB36 ... I think. I have National grid references in the
>> format "SH1234512456" that I need to convert.
>>
> 
> You have to pre-process your grid references in order to get Easting and
> Northing values in metres from the false origin of the grid. The
> 2-letter code "SH" basically means that you have to add 200000 metres to
> your Easting value 12345 and 300000 metres to your Northing value of
> 12456. Once the pre-processing is done, you can use cs2cs as suggested
> above, e.g.:
> 
> $ echo $((200000 + 12345)) $((300000 + 12456)) |
> cs2cs -v -f "%.6f" +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
> +x_0=400000 +y_0=-100000 +ellps=airy
> +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +to
> +proj=longlat +datum=WGS84
> 
> # ---- From Coordinate System ----
> #Transverse Mercator
> #       Cyl, Sph&Ell
> # +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
> # +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489
> # ---- To Coordinate System ----
> #Lat/long (Geodetic alias)
> #
> # +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
> 
> -4.777201       52.677222 52.494866
> 
> For the individual offsets by 2-letter code, you could have a looks at:
> http://www.ordnancesurvey.co.uk/oswebsite/support/knowledgebase/calculating-distances-using-national-grid-references.html
> 
> 
> Hermann
Thanks for this, I think I've got it sorted. :)